The exon-exon junction complex provides a binding platform for factors involved in mRNA export and nonsense-mediated mRNA decay

We recently reported that spliceosomes alter messenger ribonucleoprotein pa rticle (mRNP) composition by depositing several proteins 20-24 nucleotides upstream of mRNA exon-exon junctions. When assembled in vitro, this so-call ed 'exon-exon junction complex' (EJC) contains at least five proteins: SRm1 60, DEK, RNPS1, Y14 and REF. To better investigate its functional attribute s, we now describe a method for generating spliced mRNAs both in vitro and in vivo that either do or do not carry the EJC. Analysis of these mRNAs in Xenopus laevis oocytes revealed that this complex is the species responsibl e for enhancing nucleocytoplasmic export of spliced mRNAs. It does so by pr oviding a strong binding site for the mRNA export factors REF and TAP/p15. Moreover, by serving as an anchoring point for the factors Upf2 and Upf3, t he EJC provides a direct link between splicing and nonsense-mediated mRNA d ecay. Finally, we show that the composition of the EJC is dynamic in vivo a nd is subject to significant evolution upon mRNA export to the cytoplasm.